Tunable ultra-narrowband grating filters in thin-film lithium niobate

TL;DR

This paper demonstrates ultra-narrowband, tunable optical filters in thin-film lithium niobate using phase-shifted Bragg gratings, achieving high extinction ratios, extremely narrow bandwidths, and electrooptic tunability suitable for integrated photonics applications.

Contribution

It reports the implementation and optimization of phase-shifted Bragg gratings in thin-film lithium niobate, enabling reconfigurable, ultra-narrowband on-chip optical filters with record tunability and low footprint.

Findings

Achieved transmission bandwidths as narrow as 14.4 pm and 8.8 pm.

Demonstrated a tunability of 25.1 pm/V and modulation of 1.72 dB/V.

Filters have footprints smaller than 1 μm x 1 mm with up to 25 dB extinction.

Abstract

Several applications in modern photonics require compact on-chip optical filters with a tailored spectral response. However, achieving sub-nanometric bandwidths and high extinction ratios is particularly challenging, especially in low-footprint device formats. Phase shifted Bragg gratings implemented by sidewall modulation of photonic nanowire waveguides are a good solution for on-chip narrowband operation with reasonable requirements in fabrication and scalability. In this work we report on their implementation and optimization in thin film lithium niobate, a photonic platform that affords reconfigurability by exploiting electrooptic effects. The phase-shifted Bragg grating filters have a footprint smaller than 1 {\mu}m x 1mm and operate at telecom wavelengths, featuring extinction ratios up to 25 dB. We demonstrate transmission bandwidths as narrow as 14.4 pm ($Q = 1.1$ x $10^5$) and 8.8 pm ($Q = 1.76$ x $10^5$) in critically coupled structures and multi-wavelength Fabry-Perot configurations, respectively, in full agreement with theoretical predictions. Moreover, by taking advantage of the strong electrooptic effect in lithium niobate, in combination with the tight light confinement of nanophotonic wires and the ultranarrow spectral resonances of optimized grating structures, we demonstrate a tunability of 25.1pm/V and a record modulation of the filter transmission amounting to 1.72 dB/V at CMOS voltages. The results pave the way for reconfigurable narrowband photonic filters in lithium niobate with small footprint and low consumption, to be exploited towards on-chip quantum and nonlinear optics, as well as optical sensing and microwave photonics.